The usual method of preparing metal and metal alloy ribbons and the like is a chill block melt spinning method. In this procedure, the molten metal or an alloy thereof is applied to a cool, rapidly moving, thermally conductive substrate, the molten metal is rapidly cooled to below its solidification point and is then flung or forcibly removed from the substrate. The apparatus commonly used in performing this method are spinning wheels and discs in combination with either free jet or planar casting melt delivery systems. The resulting product is usually a straight or helical ribbon, tape or film of about 1 to about 2 mils in thickness. Other methods which have been used for the production of such cast metal or metal alloys include the drawing of thin filaments into a cooling medium or the generation of small diameter splats or particles.
The continuous casting of metal and alloy ribbons requires that heat be removed from the thermally conductive substrate so that it can function as a heat transfer surface or the metal or metal alloy being cast. Various single roll and double roll methods have been developed but can be operated for only extremely short periods of time if no cooling is provided. The heat can be removed from the casting substrate, either from the casting surface or from any other convenient surface of the substrate. It is necessary that the heat removal from the casting substrate be accomplished with a heat transfer coefficient sufficient to maintain the casting surface temperature below the metal or metal alloy solidification temperature. For example, cooling a molten metal rapidly (10.sup.5 -10.sup.6 .degree. F./sec.) on a 10-inch copper wheel rotating at 1500 r.p.m. (65.4 ft./sec. tangential velocity or about 321 g's angular acceleration) gives rise to a heat flux associated with the solidification which is estimated to be about 2.times.10.sup.6 BTU/hr. ft.sup.2. A 40-inch diameter copper-chrome cylinder of about 1/2 inch in thickness which rotates at 400 r.p.m. (70 ft./sec. tangential velocity or about 91 g's angular acceleration) can be subjected to an average heat flux of 370,000 BTU/hr. ft.sup.2. In these examples heat transfer coefficients of 30,000 and 5,000 BTU/hr.-ft.sup.2 -.degree. F. would typically be desired.
If the casting of the metal or metal alloy is performed under vacuum conditions, a procedure is necessary to steadily remove heat from the non-casting surface of the rapidly moving, thermally conductive substrate such as, for example, the interior of a casting wheel. Single phase forced convection has frequently been employed for cooling non-casting surfaces, such as the interior surface of a casting wheel, but this method is characterized by high volume flow rates, axial variations in heat transfer and flow maldistribution. It also requires expensive machining of the heat transfer substrate. The continuous dressing of the casting surface and the occasional machining which may also be required results in substrate thicknesses which vary over time. As a result, the cooling method which is used to maintain constant casting surface temperatures must be adequate to compensate for the varying heat transfer substrate thicknesses and must also have axially invariant heat transfer coefficients and not require elaborate machining.
Pool boiling has commonly been applied to the cooling of stationary heat sources such as nuclear and chemical reactors and power electronics, and more recently to rotating equipment. Rotating boilers seek to use higher critical heat flux (CHF) limits which are associated with the radial acceleration to reduce boiler size and to exploit an as yet unproven improvement in heat transfer coefficient. Rotating heat pipes generally utilize the acceleration as a substitute for a wick, and improve heat transfer coefficients by maintaining thin fluid layers in the evaporator and condenser sections.
It is an object of this invention to provide a method for the cooling of amorphous metal and metal alloy casting substrates, such as wheels, which is operative on the non-casting surfaces, such as the inside of casting wheels, and which has uniform axial heat transfer coefficients.
It is also an object of this invention to provide a cooling method which does not require complicated machining of the casting substrate and which can be used, if desired, to vary the casting surface temperature.
A further object of this invention is to provide a boiling heat transfer method which uses a rotating system to advantage in increasing critical heat flux limits.